Photonic crystal microcavities exhibiting cavity quantum electrodynamic (QED) phenomena can be used to construct optical devices such as high-efficiency light emitting diodes, low-threshold lasers, and single photon sources. During the growth process of a solid-state microcavity, a single narrow-linewidth emitter (quantum dot) can be embedded in the device, enabling cavity-field interaction with the quantum dot. The coupling between the quantum dot and the electric field in the cavity enhances the radiative transition rate of the quantum dot. The coupling is enhanced by a strong electric field intensity located at the quantum dot. The coupling is also increased if the volume of the fundamental electromagnetic mode in the cavity is small. Thus, for many applications of microcavities, it is desirable that the microcavity has a high quality factor (Q) and a low mode volume (V) for the fundamental mode. In other words, it is desirable that the ratio Q/V is large. For example, increasing Q/V can lead to a reduction in laser threshold. Controlling the Q/V is also useful in single-photon sources for enhancing the coupling efficiency of the dot to an output mode of interest.
A standard microcavity is the cylindrical micropost design, as illustrated in FIG. 1a. The micropost microcavity has a spacer region 100 positioned between two dielectric mirrors, a top mirror 110 and a bottom mirror 120. One or more quantum dots, or one or more quantum wells 180 are embedded at the center of the spacer region 100. The dielectric mirrors 110 and 120 are distributed Bragg reflectors (DBRs) made by stacking quarter-wave thick layers of alternating high index (nh) material 130 and low index (n1) material 140. In fabricating the device, alternating high and low index quarter-wave layers are deposited, and the spacer region 100 is made by increasing the thickness of a high index layer to a full wavelength (λ/nh). Presently used quantum dots or quantum well materials are required to be embedded in the high refractive index material (for example, InxGa1-xAs quantum dots or quantum wells embedded in GaAs, with emission wavelength typically ranging from 900 nm to 980 nm, or InxGa1-xAsyN1-y quantum wells embedded in GaAs, with emission wavelength ranging from 1300 nm to 1550 nm). The spacer region 100 is thus preferably a high-refractive index material. To maximize the quality factor, the spacer normally is designed to have a thickness of one wavelength (λ/nh). Light at the device operating wavelength λ is confined to the structure by the combination of distributed Bragg reflection in the longitudinal (vertical) direction, and total internal reflection in the transverse (horizontal) direction. The electromagnetic mode of interest is the fundamental (HE11) mode.
FIG. 1b is a graph of refractive index and corresponding electric field intensity along the longitudinal length of the device shown in FIG. 1a. The electric field intensity is a maximum 160 at the center of the high refractive index spacer region 150. This device, therefore, advantageously combines a high-index spacer and maximum field intensity at the center of the spacer where the active layer (quantum dot or quantum well) is located. However, although the Q factor for this design is high, it has the disadvantage that the mode volume V is large due to the wavelength-thick spacer region, and this large mode volume offsets the high quality factor.
To reduce the mode volume, one could design an alternative micropost microcavity as shown in FIG. 2a. The micropost of FIG. 2a has a high-refractive index spacer region 200 whose thickness is a half-wavelength. An active region 280 comprising, for example, quantum dots or quantum wells, is embedded in the spacer region 200. As with the device of FIG. 1a, the spacer region 200 is sandwiched between top 210 and bottom 220 DBR mirrors made of quarter-wave stacks of alternating high refractive index material 230 and low refractive index material 240. FIG. 2b is a graph of refractive index and corresponding electric field intensity for the design of FIG. 2a. As illustrated in the graphs, the electric field intensity is at a minimum 260 at the center of the high-refractive index region 250. Thus, although the mode volume is smaller in this design, the electric field intensity is at a minimum where the active layer is located. Because the electric field will not interact with the active layer, this design is not useful. Moreover, it is impossible to relocate the active layer to the field maximum, as the maximum is in the low index material.
To obtain an electric field maximum in a half-wavelength spacer, one could design an alternative micropost design as shown in FIG. 3a. Like the design of FIG. 2a, this design has a half-wavelength spacer 300 sandwiched between DBR mirrors 310 and 320 made of quarter-wave stacks of high index 330 and low index 340 materials. The spacer 300 in this design, however, is made of a low index material. As a result, the electric field has a maximum 360 at the center of the low index region 350, as shown in FIG. 3b. Although this design provides an electric field maximum at the center of a half-wavelength spacer, the spacer material has a low refractive index. Because the active layer (e.g., quantum dot or quantum well) needs to be embedded in the high refractive index material, as explained previously, this design is not useful.
In summary, although it is possible to achieve a maximum field intensity at the center of a high-index spacer, as shown in the graph of FIG. 1b, this device has a large mode volume. If the mode volume is decreased by using a half-wavelength spacer, however, the resulting device either has a minimum field intensity at the center of the spacer, as shown in FIG. 2b, or has a spacer with a low refractive index that is not suitable for an embedding active layer, as shown in FIG. 3b. Thus, according to conventional design principles known in the art, it is not possible to obtain a micropost microcavity device that has a maximum field intensity at the center of a high-index spacer whose thickness is half a wavelength, i.e., it is impossible to locate an active layer at the field maximum of the device with half-wavelength spacer (of either low or high-index). Consequently, although it is desirable to design microcavity devices with higher Q/V values, the design trade-offs have prevented the realization of this goal.